# Red but not dead : Unveiling the Star-forming Far-infrared Spectral Energy Distribution of SpARCS Brightest Cluster Galaxies at 0 < z < 1.8

**Authors:** N. R. Bonaventura, T. M. A. Webb, A. Muzzin, A. Noble, C. Lidman, G. Wilson, H. K. C. Yee, J. Geach, Y. Hezaveh, D. Shupe, J. Surace  
**arXiv:** [1704.02721](https://arxiv.org/abs/1704.02721)  
**Date:** 2017-04-10  
**Summary page:** https://www.summarizepaper.com/en/paper/1704.02721v1/

## Key points

- Results of a comprehensive Spitzer/Herschel infrared photometric analysis on the largest and highest-redshift sample of Brightest Cluster Galaxies (BCGs) presented
- BCGs part of the SpARCS totaling 716 galaxies with redshift up to z=1.8
- Study identifies key sources contributing to infrared energy output in BCGs across multiple redshift intervals
- Estimates for physical parameters derived from stacked {\nu}L{\nu} SEDs challenge conventional wisdom on BCG evolution
- Discovery of ongoing star formation activity in BCGs down to z=0.5 challenges prevailing beliefs on their evolutionary paths
- Star formation in BCGs attributed to major and minor 'wet' mergers rather than traditional cooling flows, supported by observational and simulation-based studies

## Summary

The authors present the results of a comprehensive Spitzer/Herschel infrared photometric analysis focusing on the largest and highest-redshift sample of Brightest Cluster Galaxies (BCGs) to date. The BCGs under investigation are part of the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS), totaling 716 galaxies with a redshift reaching up to z=1.8. This study sheds light on the dominant physical mechanisms driving the stellar-mass growth in these unique galaxies, which hold the title of being the most massive entities in the Universe and are exclusively located at the cores of galaxy clusters. By conducting a meticulous comparison between stacked broadband infrared spectral energy distributions (SEDs) of BCGs and various SED model templates available in scientific literature, key sources contributing to their infrared energy output across multiple redshift intervals spanning from 0 < z < 1.8 are identified. Estimates for several physical parameters of BCGs are derived from their stacked {\nu}L{\nu} SEDs, leading to a groundbreaking inference that challenges conventional wisdom. Contrary to prevailing beliefs suggesting that BCGs evolve passively through gas-poor minor mergers since z~4, this study unveils a star-forming population within these galaxies producing substantial amounts of stellar mass annually even down to z=0.5. This discovery aligns with an advanced semi-analytic model proposed by Tonini et al. (2012), which predicts ongoing star formation activity in BCGs throughout the considered epoch. The observed star formation inferred from stacked infrared SEDs is attributed to both major and minor 'wet' (gas-rich) mergers rather than cluster cooling flows traditionally associated with BCG star formation processes. This interpretation is supported by a lack of definitive signatures linking BCG star formation solely to cooling flows as well as corroborative evidence from observational and simulation-based studies endorsing this alternative scenario. Overall, this study not only expands our understanding of Brightest Cluster Galaxies but also challenges established paradigms regarding their evolutionary paths, highlighting the dynamic nature of these massive galactic structures within galaxy clusters.